ROCKS

Rocks are divided as follows into three great groups based on the origin acd mode 'of formation.

1.  Igneous Rocks: Those that have consolidated at or relatively near the surface of the earth from molten material called magma originating from within the earth.

2.  Sedimentary Rocks: Those produced chiefly by the breaking up of pre-existing rocks and deposition of the broken material in the form of layers.

3.  Metamorphic Rocks: Those produced from pre-existing rocks by the action of high temperatures and pressures.

Igneous Rocks

Igneous rocks are formed by the cooling of molten material (called magma) at or relatively near the surface of the earth. Magma is a naturally formed mixture of molten rocks and minerals deep down the earth. The constituents of magma are mostly complex silicates and oxides of iron, aluminium, magnesium, calcium, sodium and potassium. Silica is always present in 35 to 75%. Most of the constituents of magma are non-volatile with fusion temperature above 1000° C while some others, small in amount, are of a highly volatile character. Under the pressure of the earth's rocks the magma travels to zones of lesser pressure and along weaker planes and fissures in the existing rocks. When it erupts at the surface it is called lava as in the case of a volcano.

Rocks formed from the same magma have different names depending upon the crystalline texture which depends upon the rate of cooling which, in turn, depends upon the place of cooling, i.e, whether at depth or at surface. The rocks which have cooled at depths had very slow rate of cooling resulting in large coarse crystals and are called plutonic, those that cooled at intermediate depths had a comparatively faster rate of cooling yielding smaller size crystals and are called hypabyssal; those that cooled at of very near the surface had a fast rate of cooling yielding no crystals but nearly glassy texture and are called volcanic.

The igneous rocks are found in batholiths, lopoliths, laccoliths, dykes, sills and lava flows near volcanoes.

Batholith: It is a large igneous rock mass of irregular outline widening downwards, and without known floor (Fig 1.2).

Lopolith; It is a large igneous rock mass which differs from the batholith in that the former has a basin-like shape and a gradually decreasing width with increasing depth.

Laccolith: It is an igneous rock formed from cooling magma which caused the previous beds to arch in the from of a dome. It is much smaller than batholith, has a known floor and a domed top.

Fig 1-2.   Modes of occurrence of igneous rocks.

B—Batholith; Lp—Lopolith; Lc—Laccolith

Dyke: It is a more or less parallel-sided vertical wall of igneous rock formed due to upward intrusion of molten magma generally through fissures or cracks existing in the previous rocks, (fig. 1.2). It may be only a few metres or hundreds of metres thick, and is a common occurrence in the coal fields, e.g. the dolerite dykes and mica peridotite dykes in most of the coalfields. The coal around the dyke is usually semi-burnt, hard and useless. In some cases, however, coal near the dyke is converted into better quality like anthracite, or into naturally occurring coke which is useful. Dykes stand out as prominent ridges extending over long distances when the comparatively softer surface is eroded. An example of such dyke, seen on the surface, is Salma dyke in the Ranigunj field which extends from Damra to the West of Kalipahari and beyond. In the Jharia field, a prominent dyke runs along Telmucha, Pipratand, Phularitand and Tundu Metal Works, and another runs more or less parallel to the above, but west of Madhuban village across Jamunia stream.

Sill: A Sill is a more or less horizontal sheet of igneous rock extends laterally into the strata, usually intruding between the bedding planes. The thickness may range from a few metres to hundreds of metres. A sill is generally fed by a dyke and is rarely exposed at the surface by erosion of rocks above it.

Lava flows: The molten magma erupting to the surface is known as lava. The rock formed after cooling of lava is termed simply lava. The rocks to the south of the Satpura range in India, known by the name Deccan Trap, are lavas nearly 300 m thick covering about; 3,00,000 sq. kilometres.                                                                   

The lava flows, as they erupt to the surface, are also called extrusive igneous rocks, and batholith, laccolith, lopolith, dyke, sill, etc. are called intrusive rocks.

Composition and Classification of Igneous Rocks

Igneous rocks arc commonly classified on the basis of their chemical composition. The essential minerals that form the igneous rocks are quartz, mica (muscovite and biotite), felspars (orthoclase and plagioclase), and the ferromagnesian minerals (augite, hornblende and olivine).

On the basis of the silica content, the igneous rocks are classified as acid rocks or basic rocks as follows:

Silica more than 65%

Silica 55% to 65%

Silica 45% to 55%

Silica less than 45%

—    Acid rock, e.g. granite,

—    Intermediate rock, e.g. diorite

—    Basic rock e.g. basalt.

—    Ultra-basic rock,e.g. Peridotite

The following table shows the classification of common types of igneous rocks based on texture and chemical composition.

Acid Intermediate Basic Ultrabasic

Plutonic Granite Syenite Diorite Gabbro Peridotite

Hypabyssal Granite Syenite Diorite Dolerite

porphyry porphyry porphyry

Volcanic Obsidian Trachyte Andesite Basalt

Granite,   granite   porphyry   and   obsidian   have   the   same composition but the crystalline texture varies.
The common igneous rocks are:

Granite: It consists of large sized crystals of quartz and orthoclase felspar with some biotite (black mica). Greenish crystals of hornblende also may occur. Granite is the most common rock of the plutonic igneous type.

Very coarse granites are called pegmatites and are characterised by large crystals, sometimes 0.3 m across, e.g. at the mica mines near Kodarma (Bihar).

Syenite and Diorite: These are not deep-seated but hypabyssal rocks. These are darker than granite but have no quartz and contain felspar and biotite or hornblende.

Gabbro: This is much darker and heavier than granite containing a large propotion of ferromagnesian minerals, such as olivine, magnetite, etc. Dolerite is very widely distributed in dykes and sills and has the same composition as that of gabbro.

Peridotite: It is a common example of ultra-basic rocks and consists of ferromagnesian minerals, chiefly olivine with some augite and hornblende. Peridotites are crystalline, of very dark colour and with a Sp. gr. of 3 or over. The rock weathers easily. Mica-peridotite dykes and sills are common in Jharia, Raniganj and Karanpura coalfields.

Sedimentary Rocks

Sedimentary rocks include all rocks formed by deposition in beds or layers (strata) of the material derived from older rocks and also cover rocks formed by chemical or organic agencies. The materials may be carried by water, wind or glacier, e.g., in the case of sandstone, shale, etc. Majority .of the sedimentary rocks are formed by deposition on riverbed, or seabed and into basins or depressions on earth's surface. The consolidation of loose sediments into hard rocks is effected by pressure resulting from gradual accumulation and also by deposition of cementing materials into spaces between particles of the sedments. The chief cementing materials are calcite, quartz and iron compounds. The colour of the sedimentary rocks is usually determined by the nature of the cementing material or by some mineral finely spread throughout the rock. The colour of the rock often changes on account of alteration of the original constituents due to atmospheric agencies, chiefly water. The sedimentary rocks were formed practically in the horizontal position but due to earth movements associated with earthquakes or other causes they were tilted from the horizontal position.

Alluvium is a loose or unconsolidated deposit resulting from the breaking up of the bed rock. The broken particles either remain in position or are moved by surface water and are redeposited elsewhere as a more or less loose deposit which may convert into unconsolidated rock. The Indo-Gangetic plain has a thick layer of alluvium, several thousand metres thick.

Some sedimentary rocks are formed from plant or animal remains e.g., coal and some form of limestone. There are some sedimentary rocks which are formed by deposition of material which was once in solution in water but later on separated from it, e.g., some deposits of limestone, ironstone (hematite), gypsum, etc. Where the water carrying carbonates in solution escape drop by drop from the rock in cavities or fissures, the carbonates may deposit in the form of rods or columns of rock. If the deposit is on the floor it is known as stalagmite and it" it is hanging from the roof ii is known as stalactite.

Placers are friable sandy-clay, sandy-gravel or similar formations containing some rare  metal or mineral such  as gold, platinum, tinstone, etc.

The rocks adjoining a seam but below it are called flour of the seam and those above it are called roof of the seam. In the metal mining practice the rocks adjoining an ore body are called its wall rocks, the hanging wall being above the ore body and the foot wall being below it.

Bedding Plane: In a sedimentary rock, a bedding plane is the junction plane of one bed of rock with another. Bedding planes are therefore the surfaces which divide the different layers of water-deposited or wind-deposited sediments and are planes of weakness in the rock as the coherence between different beds is usually less than that within a bed. Splitting is therefore usually easy along a bedding plane. Bedding planes are a feature common to all sedimentary rocks and to some metamorphic rocks which were derived from them.

Dip and Strike: The strike of a bed is a level line on its surface. In effect it is a contour line in the plane of the bed. The line of true dip in a bed or surface is the steepest line in the inclined surface and is always at right angles to the strike. (Fig, 1.3). The dip of a bed along any direction between the strike and the true dip is called its apparent dip.

Unconformity: When rocks are deposited ona above the other in uninterrupted succession, they are said to be conformable. When they are laid down upon the eroded surface of older strata, the two series of strata are said to be unconformable and the plane of contact between them is called unconformity. Unconformity is evident chiefly by the difference of dips between underlying and overlying series of rocks and also by denuded and eroded surfaces of older series and presence of conglomerate at the base of upper series. (Fig 1.4)

The common sedimentary rocks are:

Conglomerate: It consists of rounded pebbles embeded in a finer grained material forming the rock. Conglomerates occur in lens-shaped masses and not in the form of regular beds. If the pebbles are not rounded but angular, the rock is called breccia.

Sandstone: This consists of particles of quartz cemented together, usually by silica, and sometimes by calcite or other cementing materials. .The colour of the sandstone depends upon the cementing materials. Sandstones are porous in nature and permit water or other fluids like gases to flow through the pores. The rocks are of common occurrence in the coalfields.

Shale: This consists chiefly of clay and has fine texture. It can he split along the bedding planes and also into thin leaves of one millimetre or so.

Fireclay: It is a variety of clay described earlier. It is a refractory material, i.e. one capable of resisting very high temperatures without becoming soft or plastic.

Laterite: Laterite is a red ferruginous porous clay, formed by weathering of such rocks as basalt, mica schist and gneisses and composed chiefly of alumina and iron oxide. It is found up to a depth of nearly 3 m in the eastern part of Raniganj and Talchir coalfields. Laterite is the common surface rock in iron ore localities.

Limestone: It consists chiefly of CaCO₃ (Carbonate of lime) with small impurities. Some limestones are made up of shells from organic origin; others are formed due to deposition of calcite from solutions. Chalk is a soft porous variety of limestone.

One of the occurrences of limestone is in the form of sea sand and near Dwarka (Gujarat) such sea sand is dredged out from the sea bed and used for the manufacture of cement. It is soft compared to the sand formed out of quartz.

Dolomite: It is a compound of CaCO₃ and MgCO₃ in various proportions.

Fossils

Fossils are the remains or traces of former living creatures or plants now preserved in rocks. Although the earth is estimated to be at least 4000 million years old, man's footprints and bones do not appear in the fossil record until about one million years ago. The remains of plants or creatures are mostly in decayed or decomposed state. Bones, teeth or shells of animals, the wood or leaves of trees, and sometimes even the footprints or tracks of animals, are preserved as fossils. Such fossils are generally found in fine grained sedimentary rocks such as limestone, shale, etc. They are not found in igneous rocks, though bedded volcanic tuffs (fragmentary materials ejected by volcanoes) may occasionally contain some fossils. To the geologist fossils are useful as their study gives a clue to:

1. The climate of the period during which the enclosing rock was laid down.

2. The nature of the rock, whether it is a deposit m fresh water, sea or on land.

3. The relative age of the rocks as rocks belonging to a particular period extending over thousands of years are characterised by the same fossils over a wide area.

4. The geography of past ages, i.e., the situation of ancient continents, seas, etc.

5. The evolution of life throughout geological time.

Metamorphic Rocks

Metamorphic rocks are altered forms of pre-existing rocks which might have been igneous or sedimentary. The alteration may be due to either heat or pressure or by both acting together. Under the effect of pressure and temperature certain of the original minerals are no longer stable and give place to new minerals. Quartz, felspar, mica and hornblende are stable under both igneous and metamorphic conditions. In addition to new minerals, new textures arise during the process of alteration or metamorphism. Rocks not altered beyond recognition include quartzite (from sandstone) and slate (from clay). Rocks altered beyond recognition include rocks known as gneisses and schists.

The original rock-forming material and the names of original rocks and metamorphic rocks produced by it are given below:

Mineral

Sand

Clay

Calcium Carbonate

  

Original rock

Sandstone

Shale

Limestone

Mudstone

Granite

Slate

Basalt

Metamorphic rock

Quartzite

Slate, Mica-schist

Marble

Argillite

Gneiss

Schist

Amphibolite